Method of heating by induction



Sept. 13, 1966 SEULEN T 3,272,954

METHOD OF HEATING BY INDUCTION Filed Sept. 25, 1964 United States Patent 3,272,954 METHOD OF HEATING BY INDUCTION Gerhard Seulen, Remscheid, and Hermann Kuhlbars, Wuppertal-Elberfeld, Germany, assignors to Deutsche Edelslahlwerke Aktiengesellschaft, Krefeld, Germany, and Allgemeine Elektricitats Gesellschaft, Berlin- Grunewald, Germany Filed Sept. 25, 1964, Ser. No. 399,303 Claims priority, application Germany, Sept. 25, 1963,

. D 42,556 3 Claims. (Cl. 21910.43)

The present invention relates to a method of heating by induction.

It is known that metal workpieces can be heated with with the aid of sevaral inductors, and in this connection it is also known that this cannot be generally done unless the several inductors are all fed from the same source.

One known method of heating cylindrical work is to rotate it so that its surface moves underenath an inductor. This method is used for instance for heating the bearing surfaces of crankshafts before they are hardened by subsequent quenching. A heating conductor loop is so positioned that the loop embraces no more than half the circumference of the treated surface. As soon as the continuously revolving bearing surface reaches hardening temperature the inductor is switched off and the work is quenched with water, causing the surface to harden.

This overall surface hardening technique is applied to the crankpins and main bearing surfaces of crankshafts and, if hardening is performed in the classical manner, that is to say if the hardened zone extends to the points where the radii at the ends of the bearing surfaces begin, then the task is easy to accomplish with the aid of conventional apparatus. An inductor is used in the form of a conductor loop comprising axial and peripheral branches which provide heating effects suitably controlled, for instance by magnetic yokes mounted on the conductors at appropriate point-s, for a hardening zone of the desired width and depth of penetration to be obtained.

However, problems arise when it is desired to harden the bearing surfaces including the radii at each end, either for the purpose of providing surfaces which can take up the thrust of the shaft or of improving the reverse bending and/or torsional fatigue strength of the shaft. It is then extremely difficult to control the heating effect of the several sections of the inductor in relation to the workpiece surface in such a way that a hardened zone of uniform depth is achieved both on the cylindrical part of the surface as well as in the radii and on the webs or shoulders at each end thereof. A considerable amount of trial and error is usually needed before the inductor satisfies these requirements.

Moreover, inductors which are to produce a hardened zone which includes the radii at the ends of the cylindrical surface have a substantially higher power consumption than inductors which are required merely to harden the cylindrical surface. In other words for hardening the radii a generator supplying a substantially greater high frequency power must be available than would be required for performing a classical hardening operation involving only the cylindrical part of the surface.

The difficulties which arise in the surface hardening of crankshafts may also arise in the case of work of other geometrical shapes, including for instance work with flat surfaces. Even in such a case it is desirable to be able fully to utilise the avaialble installed power when several inductors are used.

The problem is solved according to the invention by providing at least two inductors in the form of heating 3,272,954 Patented Sept. 13, 1966 conductor loops which provide substantially closed paths for the current above the workpiece surface.

A section of the workpiece surface is, or closely adjacent sections thereof are, simultaneously exposed to the action of these heating conductor loops in such manner that air coupling between them is avoided. The inductors are supplied by diffierent sources of current having different characteristics, frequencies and/or power. For axialy revolving surfaces, particularly for the crankpin and main bearing surfaces of crankshafts, inductors are used which only partly embrace the work, preferably only a quarter of its periphery.

It will therefore be understood that rotary generators of different types and/or frequencies and power can be used. It might have been thought that the employment of two generators in this particular way would have been out of the question because the generators would be operating in parallel. Now it is a well known fact that it is extremely difficult to operate generators or converters of the kind required for the contemplated purpose in parallel without considerable trouble, especially when the operating frequencies are, as usual, in the order of 10,000 c./s. are likewise quite impossible to Work in but having different driving motors, such as a slip-ring motor in one case and a squirrel cage motor in the other, can never be paral-lelised. Generators producing alterhating currents of different frequencies, such as 2000 and 10,000 c./s. are likewise quite impossible to work in parallel.

However, it has been recognised by the inventors that it is possible to use several inductors sup-plied by entirely different generators of different characteristics for simultaneously treating the same surface section or closely adjacent surface sections of a workpiece. Surprisingly this can be successfully done and the two generators are found not to interfere with each other; heterodyne oscillations do not build up.

Apart from the advantages afforded by being able to utilise several available generators for treating the same workpiece, the features of the method proposed by the present invention have the further advantage of permitting the heating effect of the several inductors to be controlled without operational trouble simply by varying the power and/or the current and/or the voltage.

For instance, for treating the bearing surfaces of crankshafts the inductors may be so designed that one inductor can be associated with the cylindrical portion of the surface and another with the radius and shoulder at the end of the cylindrical surface. Therefore, if a more intense heating effect is required in the region of the radius at the end, then the inductor associated with the radius may be operated at higher power or higher voltage or higher current.

Another possibility is to operate the two inductor-s for overlapping periods of time and to switch them on and off at different instants of time. Varying heating effects and heating patterns can thus be achieved at will.

Another advantage afforded by the invention is that a plant laid out for instance for the surface hardening of crankshafts of a particular size can also be used for dealing with bearing surfaces of larger dimensions, provided a second inductor plant is available which can be used for energising a second inductor to which the same section of the workpiece surface can be exposed simultaneously. It does not matter at all whether the two available generators differ in frequency, voltage or other characteristics.

The method proposed by the present invention therefore permits finely differentiated heating and hardening patterns to be achieved in the treatment of crankshafts, particularly of large and maximum sizes, but also generators of completely different types to be used to assist one another in performing the work.

The same method can be analogous-1y applied to workpieces of other shapes, for instance to work with flat surfaces, plates and so forth, the procedure being as described by reference to the heating and hardening of the bearings of crankshafts.

The proposed method will be briefly exemplified by reference to the accompanying drawings in which FIG. 1 shows one method according to the invention, and FIG. 2 shows a modification.

FIG. 1 shows the crankpin of a crankshaft which it is proposed to harden, the hardened zone to extend over the entire cylindrical surface 1 as well as into the radii 2 at each end of the cylindrical pin where the latter merges into the webs 3. To this end an inductor loop 4 which provides a substantially closed current path above the work surface and which is fed through the conductors at 5 is positioned so that it faces an axial section of the cylindrical surface. Two inductor loops are positioned at 6 and these have linear conductor portions around the work. These inductor loops are fed through conductors located outside the plane of the paper. Their purpose is to heat the radii at 2. In order to prevent air coupling between the inductors, the inductors 6 are provided with laminar yokes, as indicated at 7. Generally, the provision of such laminar yokes on inductor 6 will be sufficient to prevent air coupling effects between the two inductors 6 and 4. However, it may be useful also to provide inductor 4 with laminar yokes as indicated at 8.

FIG. 2 shows the crankpin 1 of a crankshaft in perspective. It is similarly proposed to heat and harden this crankpin including the radii at each end. In this case inductor 4 is not positioned directly adjacent inductors 6 which are provided for heating the radii 2 (FIG. 2 actually shows only one such inductor). It is therefore unnecessary to provide laminar yokes for screening. In the illustrated example inductor 4 is provided with laminar yokes 9, but these are not intended for screening from inductor 6, but rather for the purpose of influencing the pattern of the heating effect.

A person skilled in the art therefore has the choice of preventing interference between the inductors and hence between the generators by air coupling effects between the inductors either by providing suitable screening by means of laminar yokes or by suitably disposing the inductors in space in relation to one another.

What we claim is:

1. A method of inductively heating a workpiece by means of at least two inductors, in which the said inductors have the form of heating conductor loops which provide substantially closed paths for the current over the workpiece surface, and while air coupling between the inductors is substantially suppressed the said inductors act simultaneously on different areas of the work surface while energised by current source-s having different characteristics frequencies and outputs.

2. The method according to claim 1 applied to the heating, incidental to hardening, of the axially extending cylindrical surfaces and rounded end junctions thereof in cr-ankshafts, which comprises rotating the crankshaft about its axis and heating a said cylindrical portion by at least one inductor loop shaped to conform substantially to and to extend partly around said portion while simultaneously heating the said rounded junctions at the end of the cylindrical portion by inductor loops situated in the vicinity of and extending partially around the said junctions.

3. The method according to claim 1, which further comprises switching off the respective inductor at different instants of time so that the simultaneous action of the inductors is accompanied by a period when at least one will be switched off and at least one active.

References Cited by the Examiner FOREIGN PATENTS 1,125,965 11/1956 France.

468,825 '11/ 1928 Germany. 857,839 12/ 1952 Germany. 756,749 3/1953 Germany. 917,201 8/ 1954 Germany.

OTHER REFERENCES Cable, Induction and Dielectric Heating, 1954, Reinhold Publishing Corp., New York.

RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner. 

1. A METHOD OF INDUCTIVELY HEATING A WORKPIECE BY MEANS OF AT LEAST TWO INDUCTORS, IN WHICH THE SAID INDUCTORS HAVE THE FORM OF HEATING CONDUCTOR LOOPS WHICH PROVIDE SUBSTANTIALLY CLOSED PATHS FOR THE CURRENT OVER THE WORKPIECE SURFACE, AND WHILE AIR COUPLING BETWEEN THE INDUCTORS IS SUBSTANTIALLY SUPPRESSED THE SAID INDUCTORS ACT SIMULTANEOUSLY ON DIFFERENT AREAS OF THE WORK SURFACE WHILE ENERGISED BY CURRENT SOURCES HAVING DIFFERENT CHARACTERISTICS, FREQUENCIES AND OUTPUTS. 